The present invention generally relates to the conversion of organic lignocellulosic material (biomass) into a useful gas-phase fuel, and more particularly to a system and method capable of scalable and continuous conversion of biomass materials into useful gas-phase fuels.
Biomass gasification is a well-known process for producing synthesis gas (syngas), which as also known in the art is a gas mixture containing varying amounts of carbon monoxide (CO) and hydrogen gas (H2). Though having a lower energy density than natural gas, syngas is suitable for use as a fuel source.
Within a biomass gasifier, a carbonaceous material typically undergoes pyrolysis, during which the carbonaceous material is heated to release volatiles and produce char. Combustion then occurs during which the volatiles and char react with oxygen to form carbon dioxide (CO2) according to the reactionC+O2→CO2 
The next process is the gasification process, during which the char reacts with carbon dioxide and steam (H2O) to produce carbon monoxide and hydrogen gas via the reactionC+H2O→H2+CO
Consequently, the biomass gasification process employs oxygen or air to combust some of the biomass and produce carbon monoxide and energy, the latter of which is utilized to convert the remaining biomass to hydrogen and additional carbon monoxide.
Various types of gasifier designs are known. The most common type of gasifier used in biomass gasification is believed to be an up-draft design (counter-current) design, in which air, oxygen and/or steam flows upward through a permeable bed of biomass and counter-currently to the flow of ash and other byproducts of the reaction. Typical up-draft gasifiers have significant technical shortcomings. First, the introduction of air into the hot gasification chamber partly combusts the biomass, yielding a lower overall heating value compared to pure gasification. Second, if air is used as the gasification agent, nitrogen in the air is a diluent that reduces the energy content per unit volume of the output gas, making the output gas inconvenient for use in gas turbines, for storage, and for subsequent chemical processing. Third, tars and phenolic hydrocarbons produced in an up-draft gasifier require removal to reduce emissions, avoid fouling of a gas turbine, and avoid catalyst poisoning when used to create liquid fuels. The removal equipment adds to system complexity and size, with the result that for economic reasons the gasifier is usually limited to large installations. Because biomass is a low-energy content fuel and is dispersed geographically, a large-scale gasifier requires transport and storage of the biomass, which negatively affects the economic payback for the system.
If biomass gasification is performed at a sufficiently high temperature, all organic material can be broken down into simple molecules, such as carbon monoxide and hydrogen, and combustion byproducts can be avoided if the organic material is broken down predominantly through anaerobic pyrolysis. The latter is significant because it reduces toxic effluents and simplifies downstream syngas clean-up requirements. As noted above, it is also advantageous to minimize the introduction of air gases, since air is predominantly nitrogen. Achieving high temperatures efficiently without the introduction of air for partial combustion requires a system design which maximizes heat transfer, minimizes heat loss, and provides for continuous operation. These advantages have been realized for small scale systems (for example, up to 10 tons of biomass per day), as reported in U.S. patent application Ser. No. 12/357,788, whose contents are incorporated herein by reference. However, it would be desirable if such advantages could also be realized with a system and process that are scalable to systems that are larger by at least two orders of magnitude, for example, about 1000 to 10,000 tons/day.